The fourth Republican debate in the search for a US presidential nominee saw Donald Trump under sustained attack from his rivals – not least over the billionaire businessman’s plan to build a wall along the border with Mexico and deport millions of illegal immigrants.

Point of view

Twelve million illegal immigrants. To send them back, 500,000 a month, is just not possible

“We are a country of laws. We need borders. We will have a wall. The wall will be built. The wall will be successful,” front-runner Trump said in Tuesday night’s debate, hosted by Fox Business in Milwaukee, Wisconsin.

“And if you think walls don’t work, all you have to do is to ask Israel. The wall works, believe me. Properly done. Believe me.”

Making more of an impression than in previous debates, former Florida governor Jeb Bush begged to differ.

“Twelve million illegal immigrants. To send them back, 500,000 a month, is just not possible,” he said.

“And it is not embracing American values. And even having this conversation sends a powerful signal. They are doing high-fives in the Clinton campaign right now when they hear this.”

A spokesman for Hillary Clinton’s campaign, Brian Fallon, tweeted during the debate that “we actually are doing high-fives right now.”

Trump also gave Bush an opening when he said it was okay with him if Russian President Vladimir Putin “wants to go and knock the hell out of ISIS” in Syria and Iraq, a reference to militants of the so-called Islamic State in the two Middle East countries.

Bush quickly interjected.

“We’re not going to be the world’s policemen, but we sure as heck better be the world’s leader,” he said, adding that Trump’s views of Putin and his policies in Syria were “like a board game. That’s like playing Monopoly or something. That’s not how the real world works.”

Bush’s steadfast performance was enough to halt the sense of desperation around his presidential campaign and may buy him time to counter the rise of his chief rival, Florida Senator Marco Rubio, who gave a mistake-free performance as did Texas Senator Ted Cruz.